LED light-emitting devices, which utilize light-emitting diodes, are used in many displaying elements of instruments such as mobile devices, PC peripheral equipments, OA equipments, various kinds of switches, light sources for backlighting, and indicating boards. The LED light-emitting devices are strongly required not only to have high efficiencies, but also to be excellent in color rendition when used for general lighting or to deliver wide color gamuts when used for back-lighting. In order to enhance the efficiencies of light-emitting devices, it is necessary to improve those of fluorescent substances used therein. In addition, from the viewpoint of realizing high color rendition or a wide color gamut, it is preferred to adopt a white light-emitting device that comprises a combination of a blue light-emitting excitation source, a fluorescent substance emitting green luminescence under excitation by blue light, and another fluorescent substance emitting red luminescence under excitation by blue light.
Meanwhile, high load LED light-emitting devices generate heat in operation so that fluorescent substances used therein are generally heated to about 100° C. to 200° C. When thus heated, the fluorescent substances generally lose emission intensity. Accordingly, it is desired to provide a fluorescent substance less undergoing the decrease of emission intensity (temperature quenching) even if the temperature rises considerably.
Eu-activated alkaline earth orthosilicate phosphors are typical examples of fluorescent substances emitting green or red luminescence under excitation by blue light, and hence are preferably used in the aforementioned LED light-emitting devices. The green light-emitting fluorescent substance of that phosphor shows, for example, luminance characteristics such as an absorption ratio of 73%, an internal quantum efficiency of 85% and a luminous efficiency of 62% under excitation by light at 460 nm; and the red light-emitting one of that phosphor shows, for example, luminance characteristics such as an absorption ratio of 82%, an internal quantum efficiency of 66% and a luminous efficiency of 54% under excitation by light at 460 nm. A LED light-emitting device comprising those in combination gives white light with such a high efficiency and such a high color gamut as to realize 186 lm/W based on the excitation light and a general color rendering index Ra=86, respectively.
However, if those Eu-activated alkaline earth orthosilicate phosphors are used in a high load LED light-emitting device, they often undergo the above-described decrease of emission intensity. Specifically, when the temperature rises, those fluorescent substances remarkably suffer from the temperature quenching but the blue LED is not so affected that the emission intensity thereof decreases only slightly. Consequently, the resultant light radiated from the device is liable to lose the balance between the emission from the blue LED and the luminescence from the fluorescent substances. Further, since the temperature quenching acts in different manners on the green and red light-emitting fluorescent substances, it often becomes difficult to keep the balance between green and red colors in the resultant light in accordance with increase of the load. As a result, there is a problem of serious color discrepancies caused by loss of the balance among the blue, green and red emissions.